a) Field of the Invention
The present invention relates to a vari-focal lens system which is to be focused by the rear focusing method or the inner focusing method.
b) Description of the Prior Art
The conventional zoom lens system is focused by moving the front lens unit along the optical axis. This is because the moving distance required for focusing varies along with variation of the focal length of the zoom lens system as a whole when the zoom lens system is focused by the inner focusing method or the rear focusing method, i.e., by moving a lens unit which is arranged on the image side of the lens unit having the vari-focal function in the zoom lens system. Accordingly, the zoom lens system which adopts the rear or inner focusing method does not allow graduation of the distances to objects to be brought into focus in correspondence to the moving distances of the focusing lens unit. Further, the conventional zoom lens system is defocused by changing focal length thereof after the zoom lens system is focused on an object located at a certain distance.
However, most of the still cameras and video cameras are equipped nowadays with autofocusing mechanisms. Therefore, the above-mentioned problem posed by adopting the rear focusing method or the inner focusing method can be corrected by using, as a focus detecting system, the so-called TTL type focusing system or the autofocusing system which is equipped with a focal length encoder for zoom lens systems, a ROM for storing lens system operating data, a calculating mechanism and a control mechanism.
Furthermore, the conventional zoom lens systems comprise a front lens unit which is generally large and heavy. Therefore, this lens unit is moved slowly for focusing, thereby slowing down focusing speed. In order to correct this problem, it is necessary to strengthen the power of the driving system though it results in a disadvantage of enhancing power consumption.
Accordingly, a merit is obtained for the autofocusing cameras by adopting the rear focusing method or the inner focusing method which performs focusing by moving a rear or intermediate lens unit which is small and light in weight. For this reason, there have been proposed numerous zoom lens systems which adopt the rear focusing method or have the compositions suited for the rear focusing method. From the mechanical viewpoint, however, it is desirable for a zoom lens system to have a composition as simple as possible.
As conventional examples of zoom lens systems each of which uses only two movable lens units for both the variation of focal length and focusing, there are known the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 62-178917 and Japanese Patent Kokai Publication No. Sho 63-29718. Each of these conventional examples consists, in the order from the object side, of a first lens unit having a positive refractive power, a second lens unit which has a negative refractive power and a vari-focal function, a third lens unit having a positive refractive power, and a fourth lens unit which has a positive refractive power, and is moved for focusing and for correcting the variation of image point caused due to the variation of focal length. Since the fourth lens unit of this conventional example has an imaging magnification lower than 1x, the zoom lens system is adapted so as to move the fourth lens unit toward the object side when the zoom lens system is to be focused on an object located at a short distance.
Moreover, a certain zoom lens system comprises a wide airspace between the third lens unit and the fourth lens unit. However, it is desired for such a zoom lens system to reduce the airspace as much as possible so as to meet the recent demand for compact design of the zoom lens system.
In addition, since the paraxial ray is high on the fourth lens unit, a zoom lens system which has a small F number or a large aperture is easily influenced in optical performance by eccentricity of lens element and can hardly be manufactured in practice. Especially, considering the fact that the image pickup devices which recently have small sizes impose strict manufacturing precision on lens elements, it is undesirable to select the rear focusing method or moving the fourth lens unit for focusing. The rear focusing method is still undesirable for the same reason even when the role of the fourth lens unit is allotted to the third lens unit.